In the past, electric power consumers ignored service interruptions of less than three seconds. With the increasing use of digital computer equipment electric power consumers are becoming less tolerant of short term voltage dips and sags. These disturbances cause flashing clock displays, resets to VCRs and personal computers, and lost production on automated assembly lines and businesses that rely on computer networks. Voltage sags and voltage dips are two closely related problems. Voltage sag is a partial reduction in the magnitude of voltage and voltage dip describes a situation wherein the magnitude of the voltage phasor drops significantly, often to zero. Voltage sags often persist for extended periods and are usually related to system loading conditions, while voltage dips generally occur for brief durations and are caused by faults on the power system. In either case, it is desirable to support the voltage to prevent service interruption and inconvenience.
One source of complaints regarding power quality are single phase disturbances on the transmission and distribution network. Most utility distribution system disturbances are single phase events. These disturbances may have relatively long durations because ground faults can require several seconds to clear on some distribution feeders. Balanced three phase disturbances are generally less common. Major three phase voltage dips are primarily due to closing into grounded equipment, but generally last only a few cycles. Less severe voltage sags occur during transient conditions following energization of large loads or during recovery from a major network disturbance. The latter events can last for a second or more. Uninterruptible power supply is a good technical solution it is expensive especially when a large load area is to be served. Accordingly it will be appreciated that it would be highly desirable to have a relatively inexpensive voltage regulator that boosts voltage during voltage dips and sags and supports the load during severe unbalanced voltage disturbances.